The present invention is directed to bicyclic compounds that are inhibitors of mammalian Target Of Rapamycin (mTOR) kinase (also known as FRAP, RAFT, RAPT, SEP). In particular, the present invention is directed to fused bicyclic compounds that are mTOR inhibitors useful in the treatment of cancer.
International Patent Publication WO 2001 019828 describes the preparation of heteroaromatic amines as protein kinase inhibitors. International Patent Publication WO 2005/047289 describes pyrrolopyrimidine compounds useful in treatment of cancer.
It has been shown that high levels of dysregulated mTOR activity are associated with variety of human cancers and several hamartoma syndromes, including tuberous sclerosis complex, the PTEN-related hamartoma syndromes and Peutz-Jeghers syndrome. Although rapamycin analogues are in clinical development for cancer as mTOR kinase inhibitor, the clinical out come with CCI-779 is just modest in breast and renal cancer patients. This is probably because rapamycin partially inhibits mTOR function through raptor-mTOR complex (mTORC1). It has been also found that ⅔ of the breast cancer and 1/2  of renal cancer patients are resistant to rapamycin therapy. With a recent discovery of rictor-mTOR complex (mTORC2) which is involved in phosphorylation of AKT (S473) that is important in regulation of cell survival and modulation of PKCα that plays a major role in regulation of actin cytoskeletal organization in a rapamycin-independent manner, and inhibition of these activities of mTOR is probably important for broader antitumor activity and better efficacy. Therefore, it is desirable to develop novel compounds that are direct inhibitors of mTOR kinase, which would inhibit mTORC1 and mTORC2.
Rapamycin, a macrolide antibiotic has been shown to specifically inhibit mTOR kinase activity in vitro and in vivo in several studies. Although precise mechanism by which rapamycin inhibits mTOR function is not well understood, it is known that rapamycin first binds to FKBP12 (FK506 binding protein) and then binds to FRB domain of mTOR and thus inhibit mTOR activity by inducing conformational changes, which inhibits substrate binding. Rapamycin has been widely used as a specific mTOR inhibitor in preclinical studies to demonstrate role of mTOR in signal transduction and cancer. But rapamycin was not developed as a cancer therapy because of stability and solubility problems even though significant antitumor activity was observed in the NCI screening programme. However, synthesis of rapamycin analogues with superior solubility and stability properties has led to run the clinical trails with CCI-779, RAD001 and AP23573. The most advanced rapamycin analogue, CCI-779 has shown modest anti-tumor activity in Phase II breast, renal carcinoma and mantle cell lymphoma clinical trials.
The Tor genes were originally identified in yeast as the targets of the drug rapamycin. The structurally and functionally conserved mammalian counter part of yeast TOR, mTOR was later discovered. mTOR is a member of the phosphoinositide kinase-related kinase (PIKK) family, but rather than phosphorylating phosphoinositides, phosphorylates proteins on serine or threonine residues. Genetic studies have shown that mTOR is essential for cell growth and development in fruit flies, nematodes and mammals, and the disruption of the genes encoding mTOR results in lethality in all species. Several studies have demonstrated that mTOR has a central role in controlling cell growth, proliferation and metabolism. mTOR regulates a wide range of cellular functions, including translation, transcription, mRNA turnover, protein stability, actin cytoskeletal organization and autophagy. There are two mTOR complexes in mammalian cells. MTOR complex I (mTORC1) is a raptor-mTOR complex, which mainly regulates cell growth in a rapamycin-sensitive manner whereas mTOR complex II (mTORC2) is a rictor-mTOR complex, which regulates cytoskeletal organization in a rapamycin-insensitive manner.
The best-characterized function of mTOR in mammalian cells is regulation of translation. Ribosomal S6 kinase (S6K) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1), the most extensively studied substrates of mTOR, are key regulators of protein translation. S6K is the major ribosomal protein kinase in mammnalian cells. Phosphorylation of S6 protein by S6K selectively increases the translation of mRNAs containing a tract of pyrimidines motif; these mRNAs often encode ribosomal proteins and other translational regulators. Thus, S6K enhances overall translation capacity of cells. 4E-BP1, another well-characterized mTOR target, acts as a translational repressor by binding and inhibiting the eukaryotic translation initiation factor 4E (eIF4E), which recognizes the 5′ end cap of eukaryotic mRNAs. Phosphorylation of 4E-BP1 by mTOR results in a dissociation of 4E-BP1 from eIF4E, thereby relieving the inhibition of 4E-BP1 on eIF4E-dependent translation initiation. eIF4E overexpression enhances cell growth and transforms cells by increasing the translation of a subset of key growth-promoting proteins, including cyclin D1, c-Myc and VEGF. Therefore, mTOR-dependent regulation of both 4E-BP1 and S6K might be one mechanism by which mTOR positively regulates cell growth. mTOR integrates two of the most important extracellular and intracellular signals involved in the regulation of cell growth: growth factors and nutrients. Growth factor, such as insulin or IGF1 and nutrients, such as amino acids or glucose, enhance mTOR function, as evidenced by an increased phosphorylation of S6K and 4E-BP1. Rapamycin or dominant negative mTOR inhibits these effects, indicating that mTOR integrates the regulation of signals from growth factors and nutrients.
Signalling pathways that are upstream and downstream of mTOR are often deregulated in variety of cancers, including breast, lung, kidney, prostate, blood, liver, ovarian, thyroid, GI tract and lymphoma. Oncogenes including overexpressed receptor tyrosine kinases and constitutively activated mutant receptors activate PI3K-mediated signaling pathways. Additional alterations of the PI3K-mTOR pathway in human cancers include amplification of the p110 catalytic subunit of PI3K, loss of PTEN phosphatase function, amplification of AKT2, mutations in TSC1 or TSC2, and overexpression or amplification of eIF4E or S6K1. Mutation or loss of heterozygosity in TSC1 and TSC2 most often give rise to Tuberous Sclerosis (TSC) syndrome. TSC is rarely associated with malignant tumors, although patients with TSC are at risk for malignant renal cancer of clear-cell histology. Although inactivation of TSC might not lead to malignancy per se, deregulation of this pathway seems crucial for angiogenesis in developing malignancies. TSC2 regulates VEGF production through mTOR-dependent and -independent manner.
With the recent discovery of rapamycin independent function of mTOR (by mTOR2) in phosphorylation AKT (at S473) that is important in regulation of cell survival and modulation of PKCα, which plays a major role in regulation of actin cytoskeletal organization, it is believed that inhibition of mTOR function by rapamycin is partial. Therefore, discovery of a direct mTOR kinase inhibitor, which would completely inhibit the function of both mTORC1 and mTORC2, is required for broader anti-tumor activity and better efficacy. Here we describe the discovery of direct mTOR kinase inhibitors, which can be used in the treatment of variety of cancers—including breast, lung, kidney, prostate, blood, liver, ovarian, thyroid, GI tract and lymphoma—and other indications such as rheumatoid arthritis, hamartoma syndromes, transplant rejection, IBD, multiple sclerosis and immunosuppression.
Recent success of Tarceva™, an EGFR kinase inhibitor for the treatment of NSCLC and prior success with Gleevec™ for the treatment of CML indicate that it is possible to develop selective kinase inhibitors for the effective treatment of cancers. Although there are several anti-cancer agents including kinase inhibitors, there is still continuing need for improved anti-cancer drugs, and it would be desirable to develop new compounds with better selectivity, potency or with reduced toxicity or side effects.
Thus, it is desirable to develop compounds that exhibit mTOR inhibition in order to treat cancer patients. Further, such compounds may be active in other kinases such as, for example, PI3K, Src, KDR, to add efficacy in breast, non-small cell lung cancer (NSCLC), renal cell carcinoma, mantle cell lymphoma, endometrial cancers, or other hamartoma syndromes.